Earlier quoted context omitted.
Not highest volume. Highest gross profit (volume * margin). And how would you ever know either of these two (volume, margin) if you were an arbitrary 3rd party? Amazon doesn't need to branch out into every single long tail product to cause severe disruption to a retail sector that is often predicated on low single digit margins.
How does Amazon know margin? Amazon know volume, Amazon goes to suppliers on alibaba and gives them the quantity they require and then Amazon figures out what margin they'll be making if they sell it at the same or lower price than the original seller.
Start with volume as a suggestion of which products to investigate for purchase price with sellers. If you can get the "right" price with the "appropriate" volume, start selling the product direct.
Also, at Amazon scale, you can estimate margin by looking at price variation over time and throwing in some well tested assumptions.